The short answer is **yes, absolutely**ΓÇöa modern 200 MHz passive scope probe on its **10X setting** is an excellent tool for this, but there are a few critical alignment and impedance caveats you need to keep in mind to prevent throwing off your measurements or detuning the oscillator.
Here is the technical breakdown of how a 10X scope probe compares to the original alignment specs and how to safely deploy it on your SX-28 bench.
---
## 1. Impedance Matching: Scope vs. VTVM
The original Hallicrafters alignment manual assumes the use of a Vacuum Tube Voltmeter (VTVM), which traditionally boasts an input impedance of $11\,\text{M}\Omega$ (typically $1\,\text{M}\Omega$ isolated right at the probe tip to prevent AC loading, feeding into $10\,\text{M}\Omega$ in the meter body).
When you switch to a modern oscilloscope:
* **The Probe Impedance:** On the **10X setting**, a standard passive scope probe presents an input resistance of exactly **$10\,\text{M}\Omega$** to the circuit under test. * **The Verdict:** $10\,\text{M}\Omega$ is incredibly close to the $11\,\text{M}\Omega$ VTVM benchmark. For the purposes of tracking DC AVC voltage or monitoring RF/IF signal peaks, the DC resistive loading variation is negligible.
## 2. Dealing with the 1 Megohm Tip Resistor Spec
The manual calls out a $1\,\text{M}\Omega$ resistor at the tip of the VTVM probe for a specific reason: to act as a low-capacitance isolation barrier. It prevents the long, shielded test lead of the meter from acting as a massive capacitive load ($\sim100\text{ pF}$ or more), which would instantly detune or kill delicate RF/IF tank circuits.
* **How your 10X probe solves this:** Inside a 10X passive probe, there is a parallel $9\,\text{M}\Omega$ resistor and adjustable compensation capacitor network. This scales down your scope's internal $1\,\text{M}\Omega$ input to match the 10:1 ratio. * **The result:** Instead of a raw wire, the circuit under test "sees" an ultra-low parasitic capacitanceΓÇöusually between **$10\text{ to }15\text{ pF}$** depending on the exact model. This is physically equivalent to (and often better than) an old-school VTVM probe with an isolation resistor. You do **not** need to add an extra external $1\,\text{M}\Omega$ resistor to the tip of your 10X probe.
---
## 3. Critical Caveats for the SX-28 Bench
While the probe is usable, using a scope instead of a VTVM introduces two factors you must account for:
### A. The 10X Voltage Reduction
Because the probe is an attenuator, it drops the voltage signal reaching your scope screen by a factor of 10.
* If you are measuring small AVC changes during alignment (which might only be $0.5\text{ V}$ to $2\text{ V}$ DC), make sure your scope channel is set to **10X attenuation mode** in the software menu so the onscreen scaling reads correctly. * Ensure your scope vertical sensitivity can resolve down to the millivolt level cleanly so you can see the absolute "peak" of the wave forms.
### B. The Ground Loop Trap (Crucial for the SX-28)
An old battery-operated or ungrounded VTVM has a floating chassis. A modern oscilloscope **shares its signal ground directly with the 3-wire mains earth ground plug**.
> ⚠️ **Warning:** Since your custom antenna/metal roof transformer topology relies heavily on a floating counterpoise, and the SX-28 chassis itself might be floating or isolated, clipping a standard scope ground lead to the receiver chassis can introduce mains hum, loop currents, or alter the RF characteristics of the deck.
* **The Fix:** If you are probing live RF signals (like tuning C7 through C12 for oscillator feedback), keep the scope ground lead clipped as close as humanly possible to the specific tube shield or local chassis point of that RF stage to minimize ground loop return paths. Avoid draping long ground leads across the chassis.
## Summary Checklist
1. Put the probe switch firmly on **10X**. 2. Change your scope channel input setting to **10X** so your voltage readouts match reality. 3. **Do not** add an extra external $1\,\text{M}\Omega$ resistor to the tip. 4. Use a very short ground lead on the probe to prevent picking up stray AC line noise.
Page: /1_need_help/Gemini_34/Gemini_34.shtml
Last modified: Tuesday, 14 Jul 2026